papermc/Spigot-API-Patches/0133-Remove-deadlock-risk-in-firing-async-events.patch

From cc0de4b8d6d4373fada1806dc617c7dc71160094 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Sun, 9 Sep 2018 00:32:05 -0400
Subject: [PATCH] Remove deadlock risk in firing async events

The PluginManager incorrectly used synchronization on firing any event
that was marked as synchronous.

This synchronized did not even protect any concurrency risk as
handlers were already thread safe in terms of mutations during event
dispatch.

The way it was used, has commonly led to deadlocks on the server,
which results in a hard crash.

This change removes the synchronize and adds some protection around enable/disable

diff --git a/src/main/java/org/bukkit/plugin/SimplePluginManager.java b/src/main/java/org/bukkit/plugin/SimplePluginManager.java
index cb2b0b9cb..a7dd902fb 100644
--- a/src/main/java/org/bukkit/plugin/SimplePluginManager.java
+++ b/src/main/java/org/bukkit/plugin/SimplePluginManager.java
@@ -385,7 +385,7 @@ public final class SimplePluginManager implements PluginManager {
      * @param plugin Plugin to check
      * @return true if the plugin is enabled, otherwise false
      */
-    public boolean isPluginEnabled(Plugin plugin) {
+    public synchronized boolean isPluginEnabled(Plugin plugin) { // Paper - synchronize
         if ((plugin != null) && (plugins.contains(plugin))) {
             return plugin.isEnabled();
         } else {
@@ -393,7 +393,7 @@ public final class SimplePluginManager implements PluginManager {
         }
     }
 
-    public void enablePlugin(final Plugin plugin) {
+    public synchronized void enablePlugin(final Plugin plugin) { // Paper - synchronize
         if (!plugin.isEnabled()) {
             List<Command> pluginCommands = PluginCommandYamlParser.parse(plugin);
 
@@ -430,7 +430,7 @@ public final class SimplePluginManager implements PluginManager {
         disablePlugin(plugin, false);
     }
 
-    public void disablePlugin(final Plugin plugin, boolean closeClassloader) {
+    public synchronized void disablePlugin(final Plugin plugin, boolean closeClassloader) { // Paper - synchronize
         // Paper end - close Classloader on disable
         if (plugin.isEnabled()) {
             try {
@@ -490,6 +490,7 @@ public final class SimplePluginManager implements PluginManager {
             defaultPerms.get(false).clear();
         }
     }
+    private void fireEvent(Event event) { callEvent(event); } // Paper - support old method incase plugin uses reflection
 
     /**
      * Calls an event with the given details.
@@ -499,22 +500,7 @@ public final class SimplePluginManager implements PluginManager {
      * @param event Event details
      */
     public void callEvent(Event event) {
-        if (event.isAsynchronous()) {
-            if (Thread.holdsLock(this)) {
-                throw new IllegalStateException(event.getEventName() + " cannot be triggered asynchronously from inside synchronized code.");
-            }
-            if (server.isPrimaryThread()) {
-                throw new IllegalStateException(event.getEventName() + " cannot be triggered asynchronously from primary server thread.");
-            }
-            fireEvent(event);
-        } else {
-            synchronized (this) {
-                fireEvent(event);
-            }
-        }
-    }
-
-    private void fireEvent(Event event) {
+        // Paper - replace callEvent by merging to below method
         HandlerList handlers = event.getHandlers();
         RegisteredListener[] listeners = handlers.getRegisteredListeners();
 
-- 
2.18.0